Th. G. Sahama

High hafnium members of the zircon-hafnon series from the granite pegmatites of Zambzia, Mozambique

A mineralogical nomenclature applicable to the natural binary isomorphous series ZrSiO4-HfSiO4 is proposed. It is suggested that (Hf, Zr)SiO4 with more than 90 mol.-% hafnium component be called hafnon.Zoned crystals ranging in composition from hafnian zircon to hafnon were found in the heavy mineral concentrates from the tantalum pegmatites of the Morrua area, Conco, Moneia and Muiane mines in the district of Zambézia, Mozambique.The density (Berman balance), Hf/Zr ratio (XRF analysis) and unit-cell parameters (X-ray powder pattern) of the samples are determined and discussed. Two microprobe analyses of hafnon from Muiane are presented.

Cookeite from the Muiane pegmatite, Zambezia, Mozambique

This paper describes a cookeite occurring as large rosettes crystallized in an open vug in the Muiane granite pegmatite in Zambezia, Mozambique. Optical properties, chemical composition, X-ray powder pattern, single crystal data and infra-red absorption are given. The mineral represents a pseudo-hexagonal one-layer semi-random cookeite with noticeable contents of beryllium and boron.

Natural boehmite single crystals from Ceylon

Single crystals of boehmite, up to 0.1 mm in size, were found in open cavities inside a corundum crystal from the Ratnapura area gem gravels in Ceylon. The unit cell parameters are (X-ray powder pattern): a0=3.695 Å b0=12.212 Å, c0=2.867 Å. The crystallographic orientation is based on X-ray single-crystal precession photographs. The crystals show the faces (it010), (001), (101), and (221). Systematic extinctions agree with the space group Amam. Optical orientation: a∥γ, b∥β, c∥α. Refractive indices are given. The mineral is optically positive with a large optic axial angle.

Niobian wolframite from the Nuaparra pegmatite, Zambezia, Mozambique

Niobian wolframite is chemically an intermediate between wolframite and an iron-manganese niobate of the type FeNbO 4 . Single crystal X-ray photographs indicate systematic extinctions consistent with the monoclinic wolfaamite space group P2/c. a 0 =4.735 , b 0 =5.726 , c 0 =5.090 , β=90°. The mineral is opaque, in colour dark grey to almost black. Specific gravity 6.08. Hardness 5. No cleavage.

Natural bismuth vanadate from the Mutala pegmatite area, Mozambique

Bismuth vanadate (microprobe test) in varying shades of orange color and in well developed crystals (averaging 0.2 mm in size) occurs in bismutite in the Mutala granite pegmatite area, district of Zambezia, Mozambique. Two modifications of BiVO4were identified. An orthorhombic form is identical with pucherite and shows a0 = 5.336 Å, b0 = 5.053 Å, c0 = 12.021 Å. The crystal habit ranges from platy to stout prismatic. The X-ray powder pattern of the monoclinic form matches that of the synthetic monoclinic Bi-orthovanadate with a0 = 5.205 Å, b0 = 11.718 Å, c0 = 5.098 Å, β = 90° 25′. The crystal habit resembles that of a pyramidal scheelite crystal with the b-axis corresponding to the scheelite c-axis. Multiple twinning is seen on (101), in some instances with a composition plane (010).

A note on crystal morphology of melilite

The paper presents a description of a long prismatic melilite which occurs in the vacuoles of a fine-grained leucite-melilite-nephelinite from the Nyiragongo volcano (Zaire).The melilite displays an exceptionally well developed morphology which reflects the point group symmetry required by the crystal structure of the mineral.

Karibibite, a new FeAs mineral from South West Africa

Abstract Karibibite (ideally, Fe 2 As 4 O 9 ) occurs in vugs in massive loellingite of the Karibib pegmatite area, South West Africa. It is brownish yellow and finely fibrous. The thickness of the soft, single fibers is less than 1 micron, unsuitable for single-crystal X-ray study. Electron diffraction and X-ray powder pattern indicate that the mineral is orthorhombic, with a 0 = 27.91 A , b 0 = 6.53 A and c 0 ( fiber axis ) = 7.20 A . The space group cannot be given. The mineral is paramagnetic with yellow fluorescence and is pleochroic with γ > 2.10, α = 1.96, 2V α large, d = 4.07. It is soluble in acids and alkali hydroxide. Decomposition starts around 320 °C. The infra-red absorption spectrum indicates absence of AsO 4 groups. The mineral is classified tentatively as an oxide or arsenite.

Genesis of the Nyiragongo lavas

Mt. Nyiragongo is one of the eight major volcanoes of the large Virunga volcanic field in the Lake Kivu area in the Eastern Congo.The lavas of Nyiragongo are rather unique. Starting from the top of the mountain, the rocks are nephelinites with some leucite and melilite. The molten material of the present-day lava lake belongs to this type of lava. Under the nephelinites, there is a thin series of leucite-rich lava beds. The main part of the volcano consists of bergalitic melitite lavas alternating with pyroclastics of similar composition.The nephelinitic material is considered to constitute the main portion of the pre-volcanic magma under the future volcano. It is pointed out that the Nyiragongo represents just the type of volcano with which the African volcanic carbonatites are associated. It is concluded that the Nyiragongo nephelinite must be interpreted in a way accepted for the Central African volcanic nephelinites in general.The bergalitic melilitite material is interpreted as a product of carbonation of the nephelinitic magma.

A note on the feldspars in the granite xenoliths of the nyiragongo magma

The lavas of the Nyiragongo volcano in Eastern Zaire contain partially fused granite xenoliths. The relictic feldspars found in these xenoliths were studied by microprobe analysis and by X-ray diffraction methods.Some xenoliths represent originally two-feldspar granites, in others only alkali feldspars or those of an anorthoclase composition were detected. All feldspars are homogeneous without perthitic textures detectable under the microscope. In thebc-diagram of Stewart and Wright all feldspars plot on the analbite-high sanidine join. These feldspars have been perfectly disordered through heating in the melilite-nephelinite magma of the volcano.